This invention relates to LED drivers, driver circuits LED circuits, and to method of operating LED drivers.
Nowadays, so-called direct-to-mains LED lighting circuits are becoming increasingly popular mainly because of form factor advantages and their low cost, although the latter depends on required performance. A linear LED driver is essentially a current source that drives high-voltage LEDs which are directly connected to the mains. Linear LED drivers are a new type of LED driver that directly connects one or multiple LEDs, or LED strings, to the mains.
FIG. 1 shows, schematically, a generic example of a “direct-to-mains” LED light circuit having a linear LED driver and the corresponding supplied voltage (Vrect) and LED string current (ILED1, ILED2 and ILED3) curve graphs during operation. Depending on the mains voltage Vmains, a varying number of LED strings may be connected in series, as shown in FIG. 1. The mains voltage Vmains is rectified by rectifier 110. As the rectified mains voltage increases, the strings LED1, LED2 and LED3 are sequentially—and cumulatively—switched on, by being supplied with current from current sources by a controller enabling, in turn, control1, then control2 then control3 as shown. The controllable current sources and the controller are typically implemented in an integrated circuit (IC)—the driver chip. As the rectified mains voltage decreases past its peak the strings are one-by-one switched off.
Some direct-to-mains LED lighting circuits suffer from unequal LED on times as is evident from FIG. 1 (see ILED1, ILED2 and ILED3), although this may be overcome by altering the switching sequence, from a “first-on-last-off” sequence to, for instance, a “first-on-first-off” sequence as disclosed in Applicant's co-pending patent application publication number US2013-0257282A1.
LED lighting circuits that are controlled with a linear LED driver suffer from 100% light ripple at twice mains frequency, as LED driving current cannot flow when the mains voltage is lower than VLED. This could result in a poor quality of light, as 100% light ripple at twice the mains frequency can be uncomfortable to some people. In order to improve the quality of light, capacitors C1, C2, and C3 may be included in parallel with respective LED string LED1, LED2 and LED3, as shown in FIG. 2. During a start-up phase, the capacitors are charged to a value which is close to the string operating voltage. Then, during normal operation, the charge stored in the capacitors can be used to power the respective LED string, reduce the current ripple and increase the quality of light. To prevent the capacitors from being discharged, diodes D1, D2 and D3 are included in the series arrangement of LED strings, between the strings.
It will be appreciated that when a lamp is powered on, the capacitors need to be charged to VLED first before the LEDs emit any light. Therefore a startup phase is required in which the capacitors are charged as quickly as possible.
When the capacitors are still discharged, the full mains voltage is across the IC and the dissipation in the current source can be high even if a low to moderate charging current is used. For example, if the rectified mains voltage is Vrect, and the voltages across the capacitors are Vc1, Vc2 and Vc3, then the voltages (Vstr1A, Vstr2A and Vstr3A) at nodes Str1A, Str2A and Str3A would be, respectively, Vrect−Vc1, Vrect−Vc1−Vc2, and Vrect−Vc1−Vc2−Vc3. When the capacitors are discharged, Vc1, Vc2 and Vc3 are all (approximately) 0, so for a charging current LED, the power dissipated in the IC would be approximately Vrect.ILED. Even for the relatively low US mains voltage of 120 Vrms the peak voltage is 170V. Depending on the physical size of the high-voltage transistors used in the driver small currents in the mA range can already lead to a significant temperature increase.
A further source of thermal power dissipation may occur during normal operation, due to the variability in the peak main voltage: the LED lamp should operate as expected at low (expected) peak mains voltage. During periods when the peak mains voltage is higher, the excess voltage is dropped in the IC containing the current source—once again this results in power dissipation which appears as heat.